(1) Field of the invention
This invention relates to the print hammer units used in character printers.
(2) Description of the prior art
The conventional fully formed character printers such as MICR or OCR character printers use a hammer unit of the type shown in FIG. 6. Such a hammer unit consists of a hammer part A which is capable of reciprocating linear motion when driven at its rear part in accordance with the print command, a supporting part C with a hole D in it for the plunger to move in, a spring G which is placed within the hole D and which provides the retracting force for the plunger to return to its normal position, and a plunger E whose tip F hits against the character to be printed when driven at its rear part B. Normally, the plunger part E is made of metallic material so as to effectively transmit the impact force from the hammer to the character to be printed. Further, at the front end of the plunger is provided a tip F made of a somewhat resilient material such as nylon resin, stc., so that the ink from the ink ribbon is transferred fully and evenly on to the paper.
Thus, in a conventional hammer unit, the tip F would wear out over long duration of use and hence would need replacement. At the time of such replacement, the conventional unit has many drawbacks as expained below.
Firstly, since the tip F and the plunger E are constructed with an integral structure, the plunger and the tip will both have to be replaced together, thus making it uneconomical.
Secondly, the plunger E is inserted int hole D in the supporting part C which is placed fixedly between the hammer A and the character to be printed. This causes the plunger replacement operation to be unusually complicated because the supporting part C and the hammer A will have to be first detached form the chassis of the unit (not shown in the figure).
Furthermore, since the plunger E is inserted in the hole D along with the spring G, the plunger very often jumps out at the time of its removal due to the force of the spring G, thus cauing additional delays in replacing the plunger.
Finally, in a conventional hammer unit of the above type, the stroke of the plunger has to be adjusted by adjusting the relative positions of the supporting part C and the plunger E and the relative positions of the supporting part C and the hammer A. As a consequence, this adujustment is very cumbersome.